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StarDate:

February 23, 2013

The units of distance that we use in our daily lives are fine for measuring things here on Earth. But when it comes to measuring distances to planets and stars, those units aren’t very useful — you need so many of them that you wind up with long strings of zeroes. So astronomers have devised special units for tabulating the distances to far-away objects.

Consider a few of the bright objects in this evening’s sky. The brightest of them all is the Moon, and not surprisingly, it’s the only one for which everyday units are fine. It’s about 240,000 miles or 385,000 kilometers away.

The next-brightest object, which is high in the sky in early evening, is the planet Jupiter. On average, it’s about 485 million miles from Earth. Astronomers use a different unit for measuring planetary distances, though — the astronomical unit, which is the distance from Earth to the Sun. Jupiter is about five astronomical units from the Sun — five times as far as Earth is.

And the brightest object after Jupiter is the star Sirius, which is in the south. It’s so far away that it takes its light more than eight years to reach Earth. Put another way, Sirius is more than eight light-years away.

Today, however, astronomers generally use another unit for measuring distances to stars and galaxies — the parsec, which is a bit more than three light-years. So Sirius is about two-and-a-half parsecs away — a distance that doesn’t require a lot of zeroes.

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